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Introduction The following information is presented to aid in a general overview of the task of system integration of a precision positioning system. Although it is possible to assemble a system capable of positioning without years of experience, it is also extremely difficult to perform all the design decisions involved without a good grounding in Physics, Mechanical Engineering, Electrical/Electronic Engineering, Computer Science, Mathematics and a healthy dose of common sense. We hope that this condensed guide will help answer some of the hundreds of questions that arise. We will be happy to help you personally with any remaining questions. (This information is a work in progress and will continue to be expanded as time permits and as the most frequently asked questions direct) System Configuration Although there are thousands of ways in which motion systems are designed, one of the most common methods is to use a rotary motor with a long screw that advances a nut as the motor turns. That's pretty simple and there are no lack of applications for such an elementary system. Electric windows in a car are one such application. There is no position feedback to the controller other than the visible position of the window, so this is called an open loop controller. It runs up or down at a fixed speed until the switch is released or until it encounters a limit switch. The first step in the direction of closed loop position control in this application is the "automatic" mode, where a single actuation of the switch is sufficient to cause the motor to run until a limit switch is actuated. The limit switch becomes a one-bit position sensor or encoder. If we were to add additional information on the exact position of the window, in the form of a position encoder, we could select the desired position and cause the motor to run until any desired position were reached, at which time we could merely remove power from the motor and allow it to coast to a stop. Windows stop pretty quickly because they have low inertia and high friction. However, this would still not be a servo system because we would not continue to compare the desired position with the actual position and drive the motor in whichever direction were required to reduce the error. If we did so, we would have a true closed loop servo system, which is what this tutorial is all about. Conceptually, high-precision positioning systems are identical to such a system as we describe. Most use a rotary motor to turn a threaded shaft through a nut, which travels back and forth as the shaft turns. The motor is connected to the shaft through some sort of coupling and usually through a gearhead that trades speed for torque. In most applications, a rotary encoder is mounted on the motor shaft to report the rotor angle of the motor. If the gearhead and coupling have an acceptable degree of lost motion or backlash, then it will be possible to position the nut on the shaft to any position which can be sensed by the encoder. The smallest increment of position that can be sensed is called the position resolution. Practically, it makes a great deal of difference how we specify the various elements of this arrangement. What sort of bearings should be used to support the load as the leadscrew nut moves it back and forth? Should we use steel, iron or aluminum for the material which mounts the various elements of the system? Does it matter? What size motor do we need? Do we lose something other than linear velocity when we use a smaller motor with a gearhead, as opposed to a larger motor with no gearhead? How much resolution do we need? Is an Oldham coupling as good as a helical coupling for high precision? What do we lose if we use plastic gears in the gearhead instead of steel? Are planetary gearheads inherently superior or inferior in some way to spur gears? Is there an advantage to any particular gear ratio? Do we really need differential line drivers for the encoder signals? Why can't we simply pick up a catalog from a well-known, reputable firm and specify exactly what we want as a complete package instead of being bombarded with all these questions that we care so little about? We are experts in our field. Why must we also become experts in motion systems integration to reach our goals?
The following comments apply equally well to rotary positioning systems as to linear systems. To preserve clarity, a linear motion system is used for illustration. The primary difference between the two being the common use of a leadscrew in linear systems and a worm screw in rotary systems to translate the rotary motion of a small motor into linear or more precise rotary motion. A secondary difference lies in the fact that there is no fixed maximum length for a linear stage, but we are confined to 3600 in rotary systems.
A typical high-precision positioning system for linear motion has several components: motor, gearhead, encoder, controller, leadscrew, bearings and platform (stage). The characteristics of many of them may overlap in equal and unequal manners. For instance, gearhead ratio and leadscrew pitch have identical effects on resolution, but different effects on dynamic performance. To begin the process of configuring a positioning system without a clear idea of the goals that must be met is a certain recipe for disaster in terms of cost and schedule. The most important step is the first one--document exactly what you need. There will be many tradeoffs and this document will help to stay on the right path. Many of the options that will be presented will have significant effects on cost and performance. The two are normally competitive, as in most things, but it is possible to achieve a good balance if only a little care is taken in selecting system components. Try not to over-specify. If an accuracy of 5 microns is sufficient, then components with 0.1 micron repeatability will only increase cost, without increasing the value of the system to the user. If the system will perform as well at an acceleration time of two seconds, then specifying 0.1 seconds for acceleration will only increase the cost of every element in the system. Accuracy and acceleration are the driving forces in system cost. It is vitally important to maintain a clear sense of the purpose of the system when facing the mountain of options and choices that are available. The primary points to be aware of are:
The effects of and importance of these criteria will be covered in the following guides. Click on the heading to be taken to the guide.
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